This invention relates to the field of crystal oscillators, and specifically to an arrangement for a fully integrated, adjustable oscillator for use with a crystal, such as an AT-cut crystal.
Temperature compensated crystal oscillators are known in the art. These crystal oscillators generally fall into two categories, namely, those having analog compensation techniques and those having digital compensation techniques. However, each of these two categories of crystal oscillators has fallen short of achieving a fully integrated oscillator arrangement for use with a crystal.
In particular, present day temperature compensated crystal oscillators utilize one or more discrete hyper-abrupt junction varactors to set the target frequency (f.sub.o) as well as to correct for variations due to shift in the crystal frequency versus temperature. As is well known in the art, this type of varactor has a linear capacitive reactance versus voltage characteristic and a large tuning ratio, or high voltage sensitivity. The significant drawback to utilizing such a hyper-abrupt varactor is that it cannot be integrated with the rest of the oscillator circuit.
By contrast, an abrupt junction varactor can be integrated onto the same semiconductor device die along with other integrated circuitry, but a device made in this structure suffers from very large "make" tolerance of as much as plus or minus 30%. Moreover, such an integrated abrupt junction varactor also suffers from a low tuning ratio, or voltage sensitivity, that ultimately limits its ability to correct for its own varactor make tolerance as well as for any fixed capacitor make tolerances and supply tolerances, and still provide enough reserve tuning for temperature compensation or aging compensation. Even though the reactance of an integrated varactor is nonlinear versus voltage, this characteristic can be compensated by utilizing an appropriate circuit.
Another problem resulting from attempting to utilize an integrated abrupt junction varactor is that very large variations in RF level can occur due to poor make tolerances. Very high RF levels limit the varactor voltage tuning range since the RF peaks must be kept from forward biasing the varactor diode junction. Very low RF levels can cause oscillations to stop as the temperature varies.